1. Field of the Invention
One or more exemplary embodiments of the present inventive concept relate to an electrophotographic toner, and more particularly, to a toner to develop electrostatic latent images.
2. Description of the Related Art
Printing devices such as printers and copiers require extreme performances. Printers that exhibit extreme performances provide full-color. Such printers have a very fast printing speed. Images printed by printers with extreme performances have excellent image quality. The printers with extreme performances are compact, inexpensive, and environmentally friendly. In order to obtain physical properties of electrophotographic toner for meeting such a trend, technology to control a shape and surface of toner particles has become more important than ever before.
As reported in N. Englert. Fine particles and human health—a review of epidemiological studies. Toxicology Letters 149 (2004), pp. 235-242, some sub-micrometer sized particles may be environmentally harmful to human bodies in outdoor and indoor environments.
In particular, ultra-fine particles (UFPs), which are nanoparticles having a typical size of 5 to less than 1000 nm, are very dangerous. Recently, among generators of UFPs produced indoor, printers have received most attention. UFPs generated by printers are affected by several factors such as structures of printers, lifespan of printers, lifespan of toner cartridges, the number of printings, types of printing media, types of toner, and the like. According to the Blue Angel eco-label for office equipment with printing function (RAL-UZ-171) standard, indoor environmental regulations on printers have been tightened and, for example, the Blue Angel label is awarded to printers only when printers discharge UFPs within a particular range.
Among properties required of toner in addition to such environmental issues, durability is related to a printing speed of printers. In general, as the printing speed of printers increases, the number of times a shearing force is exerted on toner increases. Thus, demand for high durability of toner is on the rise. To realize compact and eco-friendly printers, the amount of untransferred toner has to be reduced. In this regard, improvement of a charging uniformity and transferability of toner is required. In order to obtain high quality print images, improvements in charging stability, transferability, and cleaning ability of toner are required.
Improvements in surface characteristics of toner particles are required to give excellent charging uniformity, charging stability, transferability, and cleaning ability to the toner particles. One of the most important factors affecting the surface characteristics of toner particles may be an external additive that is added to the surface of toner particles. One of the main functions of the external additive is to help toner to maintain fluidity by preventing the toner particles from attaching to each other. External additives may also affect a charging uniformity, a charging stability, transferability, and a cleaning ability. A silica powder or a titanium oxide powder is mainly used as an external additive.
However, conventional external additives are known to be unfavorable in terms of obtaining charging uniformity. For example, fumed silica, which is the most commonly used external additive, has a very strong negative polarity. Accordingly, an excessive charge-up phenomenon may frequently occur in a toner that has fumed silica externally added thereto.
A method of externally adding titanium oxide particles, in addition to the fumed silica, to prevent excessive frictional charging due to the excessive charge-up phenomenon caused by the fumed silica has been tried. However, since titanium oxide has a low electric resistance and a good charge exchangeability, reverse or weak charged toner may be easily produced. Thus, when toner is externally added with silica, charging uniformity of the toner may be reduced.
Silica particles may be porous. In addition, silica particles may have hydrophilic surfaces. If toner that is externally added with silica particles having high porosity and highly hydrophilic surfaces is used in a high-temperature and high-humidity environment, such a toner may not be well charged due to excessive absorption of moisture, which serves as an electrical conductor. On the other hand, toner that is externally added with the silica particles is generally excessively charged in a low-temperature and low-humidity environment. That is, charging stability of toner externally added with the silica particles, which varies depending on the environment, may be deteriorated.
To address environmental charging stability degradation due to moisture, silica particles or titanium oxide particles that are surface-treated with a surface treating agent such as hydrophobic silicone oil or a hydrophobic silane coupling agent may be used as an external additive. However, if such external additive particles treated with such a surface treating agent are used, cohesiveness between toner particles increases, and thus, fluidity of toner may be rapidly degraded.
In a method of manufacturing fumed silica particles, aggregation of silica particles occurs frequently. The aggregation degrades dispersibility of the fumed silica particles. If an external additive with poor dispersibility is used, fluidity, anti-caking ability, fixability, and cleaning ability of toner obtained as a result may also be degraded.
Thus, it is difficult to provide a toner capable of addressing the UFP emission problem while having enhanced fluidity, charging uniformity, charging stability, transferability, fixability or fusibility durability and cleaning ability.